Showing papers on "Intramolecular force published in 1980"

Effective Molarities for Intramolecular Reactions

Abstract: Publisher Summary The first step toward unravelling the mechanism of an enzyme-catalyzed reaction is to specify the mechanisms available for the reaction concerned. Many of these reactions are not observed when the relevant groups are allowed to come together in bimolecular processes in aqueous solution. For mechanistic work involving intermolecular reactions, it is necessary to use activated substrates. An attractive alternative is to study intramolecular reactions. These are generally faster than the corresponding intermolecular processes. Thus, groups like carboxyl and imidazole are involved at the active sites of many enzymes hydrolyzing aliphatic esters and amides. Therefore, mechanistic studies of intramolecular reactions play an important part in elucidating the chemistry of the groups involved in enzyme catalysis and in defining the mechanisms available for particular reactions.

Investigation of exchange processes by two-dimensional nmr spectroscopy

Abstract: A new general technique for the investigation of exchange processes in molecular systems is proposed and demonstrated. Applications comprise the study of chemical exchange, of magnetization transfer by inter‐ and intramolecular relaxation in liquids, and of spin diffusion and cross‐relaxation processes in solids.

2,3-Di-propyl-1,4-dehydrobenzene

Abstract: In this paper, a detailed study of the thermolysis of (Z)-4,5-diethynyl-4-octene (4) is presented. This reaction gives high yields of products formed by rearrangement and intramolecular and intermolecular trapping of the intermediate 1,4-dehydrobenzene 5. The kinetics of the solution pyrolysis of 4 in the presence and absence of trapping agent establish that the 1,4-dehydrobenzene is a discrete intermediate on the pathway leading to products. By following this reaction in the probe of an NMR spectrometer at high temperature, CIDNP in a 1,4-dehydrobenzene reaction was observed for the first time. This observation, along with kinetic and chemical trapping evidence, indicates the subsequent formation of two additional intermediates on the pathway to products. The observation of CIDNP, coupled with the reactivity exhibited by 5 and the other two intermediates, implicate a biradical description of these molecules.

From anthracene photodimerization to jaw photochromic materials and photocrowns

Abstract: The inter and intramolecular photochemical reactions of anthracenes in the absence of oxygen is discussed. The intermolecular photodimerization of 9-substituted anthracenes in fluid solution usually leads to head-to-tail photodimers; it will be shown that this trend can be obviated by non bonding attractive interactions between substituents or by mixed photodimerization. The mechanistic aspects will be examined, next, in connection with the problem of excimer intermediacy. Bis-9-anthracenes can form photocyclomers and, in certain cases, intramolecular excimers at room temperature. Those which display interesting photochromic properties, were called "jaw compounds"; some of them induce an unusual anthracene ring cyclomerization. A study of photophysical and photochemical processes by steady state and transient kinetic analysis of ,c)-bis-9-anthryl-n-alkanes (ethane to decane) and cC ,C) -bis-9-anthryl-polyoxaalkanes allows a deeper insight into the mechanism of photodimerization. By irradiation, the latter derivatives can generate "photocrowns"; this is the first photochemical synthesis of crown-ethers.

Intramolecular vibrational energy redistribution and the time evolution of molecular fluorescence

Abstract: We note the presence of contradictory estimates of intramolecular vibrational relaxation rates in the literature where large molecules in high energy states, corresponding to huge densities of vibrational levels, have been ascribed relaxation rates orders of magnitude smaller than those assigned to smaller molecules with much smaller densities of vibrational levels. This unphysical disparity is explained as arising from vague (or undefined) definitions of intramolecular vibrational relaxation and/or from a consideration of quantities which are not directly measured or measurable. A resolution of a portion of the problem is already well known for electronic relaxation, but the application of those results to a description of the time evolution of the molecular fluorescence, produced during the intramolecular vibrational relaxation of the electronically excited molecules, requires a generalization of the electronic relaxation theory to separate and describe the ’’unrelaxed’’ and ’’relaxed’’ emission spectra. We provide this general theory of the time variation of the emission spectrum for molecules conforming to both the intermediate and statistical limits of intramolecular vibrational relaxation with emphasis placed upon the distinguishability between these two cases. The intermediate case analysis utilizes egalitarian and random coupling type models with essentially identical conclusions from both. The time evolution and relative yields associated with the emission spectra are described for both continuous and short pulse excitation, and reasons are provided for the absence of observation of time varying emission spectra in the experiments of Smalley and co‐workers. Quantum beats are possible in principle in the sparse intermediate case. Their observability depends, however, on the detection method. When the emission spectrum can be resolved, beats are expected only in the frequency integrated intensity.

Unusual metal-carbon-hydrogen angles, carbon-hydrogen bond activation, and α-hydrogen abstraction in transition-metal carbene complexes

Abstract: Alkylidene complexes of electron-deficient transition metals display an interesting structural deformation in which the carbine appears to pivot in place while the C α -H bond weakens. A molecular orbital analysis of these carbine complexes traces the deformation to an intramolecular electrophilic interaction of acceptor orbitals of the metal with the carbine lone pair. Bulky substituents on the metal and carbine protect the metal center from intermolecular reactions and control the extent of carbine pivoting. While a secondary interaction weakens the C-H α bond and attracts the a-hydrogen to the metal, full transfer of hydride to the metal is a forbidden reaction, at least for a five-coordinate, 14-electron complex. The metal-hydrogen bonding interaction guides the hydride to a neighbouring alkyl group, facilitating an α-elimination mode characteristic of the reactions of these compounds. The complexed carbene centers are unusually electron-rich, nucleophilic, by comparison with 18-electron d 6 stabilized carbene complexes. This is a consequence of an extremely effective Ta-C overlap.

Influence of starting conformations on intramolecular exciplex formation in .omega.-phenyl-.alpha.-N,N-dimethylaminoalkanes

Abstract: The intramolecular complex formation of w-phenyl-a-N,N-dimethylaminoalkanes in isopentane was investigated using stationary and nonstationary techniques. Analysis of the decay curves indicates that ground-state starting conformations influence the rate of complex formation. In deviation of the classical kinetic scheme for interand intramolecular exciplex and excimer formation, the emission of the locally excited state contains a slow-decaying component whose lifetime is longer than that of the exciplex. This slow-decaying emission is attributed to molecules in which the C-N bond has a conformation unfa­ vorable for exciplex formation. Introduction the intermolecular processes.5 The adopted scheme could be Up to now it has been assumed that intramolecular exciplex presented as shown in Scheme I. The ratio of the quantunl and excimer formation 24 follows the same kinetic scheme as yields for emission from the exciplex, cPE, and from the locally

High Asymmetric Induction in Lewis Acid‐Promoted Intramolecular Ene‐Type Reactions: A diastereo‐ and enantioselective synthesis of (+)‐α‐allokainic acid. Preliminary Communication

Abstract: The monocyclic amino diacid (+)-α-allokainic acid 1 has been prepared enantioselectively from the ester of cis-β-chloroacrylic acid and (−)-8-phenylmenthol by a series of four synthetic operations in over 15% yield. The crucial step is the intramolecular ‘ene-type’ reaction of the (Z)-diene 4 which on treatment with a mild Lewis acid undergoes a highly accelerated, dia- and enantiostereoselective cyclization to give the pyrrolidines 6 and 7 in a ratio of 95:5 (Scheme 3). Subsequent ester hydrolysis regenerates the auxiliary chiral alcohol. Similar cyclization of the (E)-diene 5 furnished a 15:85 mixture of 6 and 7 showing an efficient reversal of the optical induction by variation of the enoate geometry.

Mechanisms of unimolecular fragmentations of odd-electron cations in the gas phase: The importance of intramolecular hydrogen migration for [odd]+˙ → [even]+ transitions

Abstract: Eighteen quite different examples are discussed in which the unimolecular elimination of a given radical X˙ from an odd-electron cation cannot be described as a one-step process. Despite the fact that the radical X˙ eliminated is already present as a structural unit in the decomposing ions, the product analysis and structure determination of the even-electron ion formed, the investigation of the energetics of the reaction, the analysis of stereochemical factors, and the determination of kinetic isotope effects reveal that the eliminations must be characterized as two- or multi-step reactions. In all cases investigated, the process commences with a specific (intramolecular) hydrogen transfer to a suitable acceptor function. In this way a reactive radical site is created which induces the actual dissociation step (elimination of X˙) by simultaneous double bond formation. Due to the fact that the hydrogen migration takes place within the charge carrying part of the cation radical and that the migrated hydrogen atom remains there throughout the whole process, the intervention of a H-migration cannot be traced directly by appropriate mass shifts in the spectra of 2H labelled precursors. However, the methods mentioned above can be applied to reveal the details of the reaction mechanism, and the term ‘hidden’ hydrogen migration induced dissociation is suggested to recognize the particular rǒle of H-rearrangements to trigger radical eliminations from odd-electron cations in the gas phase.

The inter- and intramolecular photocycloaddition of ethylenes to aromatic compounds

Abstract: Dienophilic ethylenes yield solely ortho photocycloadducts with benzene whereas alkenes and enol ethers give products arising from both ortho and meta cycloaddition. The relative efficiencies of the twoprocesses are largely predictable and this combined with reasonable chemical and quantum yields make these photoreactions realistic synthetic procedures towards bicyclo[4.2.Oloctane and tricyclo[3.3.0.02'8]octane systems. The ortho cycloaddition involves a measure of charge—transfer to or from the ethylene at some stage in the reaction pathway, whereas two routes are considered to account for the orientational selectivity of the meta process involving ethylenes and substituted benzenes. Positions and modes of intramolecular reaction between the chromophores of non-conjugated phenyl vinyl bichromophoric systems are understandable in terms of their favoured molecular conformations leading to preferred orientations of the addends.